The present invention relates to the field of continuous ink jet printing and, more particularly, to improved vacuum means for use in continuous ink jet printers.
In continuous ink jet printing systems, ink from an ink reservoir is supplied under pressure to a manifold that distributes the ink to a plurality of orifices, typically arranged in linear array(s). The ink is expelled from the orifices in jets which break up into droplet streams, due to surface tension in the ink. Ink jet printing is accomplished with these droplet streams by selectively charging and deflecting some droplets from their normal trajectories. The appropriate deflected or undeflected droplets are allowed to impinge on a printing surface, and the others, not need to form the printed image, are captured by catcher means so that the fluid can be re-circulated. To retrieve the ink from the catcher means, the ink reservoir is typically under vacuum supplied by a vacuum pump.
In a typical ink jet printer, the vacuum capacity requirements (flow times pressure) are dictated by the crossflush, startup and shutdown conditions. The vacuum requirements for these conditions can require on the order of five times as high as for the normal operating condition. It is therefore necessary to control the vacuum levels in some manner.
Prior art long array ink jet printers have used single speed AC vacuum pumps to provide the required vacuum. Control of the vacuum levels has been provided by means of either air bleed means or by means to throttle the flow to the vacuum pump. These control means may include mechanical regulator valves or servo controlled flow control means, as in U.S. Pat. No. 5,394,177. With such vacuum systems the vacuum pump is operated at near rated capacity. This leads to higher than ideal wear on the pump. It also contributes significantly to the fluid system noise and cooling load. As AC pumps are typically used, one must also size the vacuum system to deal with the range of lines voltages present in the various countries. It has also been found that the controlling means, the regulator valves and the servo controlled throttle valve are prone to fail as a result of small amounts of ink mist which can be sucked out of the ink reservoir. This can result in system failures due to the improper vacuum levels which can then occur.
As discussed in commonly assigned, copending patent application Ser. No. 09/211,204, it is desirable for the ink to be heated to a temperature greater than ambient in the ink jet printing system. This elevated ink temperature improves the print window of the printhead. During operation, the catcher draws in air from the environment along with ink to be re-circulated. This air comes in at normal ambient conditions, e.g., 20 C. and 40% Relative Humidity (RH). As the air travels through the umbilical along with the heated ink, it becomes both heated and humidified. At the fluid system, the air enters the ink reservoir at approximately 40 C. and 100% RH. It then travels out from the ink tank, through the waste tank, and finally through the vacuum pumps.
While the ink reservoir is kept hot by the continual recirculation of heated ink, the vacuum lines and the waste tank are not. They are cooled by cooling fans in the fluid system cabinet so they are only slightly warmer than the ambient air. As the moist air passes through the waste tank and vacuum lines, it cools and becomes supersaturated. Water therefore condenses out on the walls of the vacuum lines. The water which forms on the walls of the vacuum lines gets dragged along or entrained by the air flow in the lines. As a result, water is carried into the vacuum pump or pumps. The entrained liquid can cause damage to the vacuum pumps in two ways. First, the liquid can occasionally be pulled into the vane area and cause hydraulic pressures to develop between the vane and the cavity wall, causing the vanes to break. Second, the water can seep into the cavity area during xe2x80x9coffxe2x80x9d periods. This can rust the rotor, cover plate and cavity. The rotor is then xe2x80x9cweldedxe2x80x9d to the surrounding parts, so that it cannot turn.
It is seen, then, that there exists a need for an improved vacuum system. One that can provide the necessary control of the vacuum levels while reducing the wear on the vacuum pump and reducing the risk of failure of the vacuum controlling means. The improved vacuum system should also protect the vacuum pumps against failure produced by condensed water in the vacuum lines.
This need is met by the present invention wherein a means is provided for preventing condensation of water in a vacuum pump used on continuous ink jet printers.
In accordance with one aspect of the present invention, a system and method are provided for protecting the vacuum pump of an ink jet printer system from the harmful effects of condensation in the vacuum lines, and from the harmful effects of moisture and water in the vacuum lines. This is accomplished by suppressing the condensation and drying the vacuum line walls.
Other objects and advantages of the invention will be apparent from the following description, the accompanying drawing and the appended claims.